Magnet resonance tomography (MRT) is an imaging method, which is above all used in medical diagnostics for representing structures and functions of tissue and organs in the body. MRT is based on the principles of nuclear magnetic resonance NMR spectroscopy. For this method, the tissue to be examined is located in a strong static magnetic field, in which the spins of atomic nuclei in the examined tissue are aligned, which results in magnetization. Through resonant excitation with an electromagnetic alternating field in the radio frequency range, the magnetization can be deflected from the direction of the static field. Due to the excitation, the spins start to precess around the direction of the static magnetic field, and the precession of the overall magnetization can be measured as a voltage signal using a coil.
When the high-frequency alternating field is switched off, the spins relax back into their initial state. For this relaxation, they require a characteristic decay time, which is typical for various elements and various compounds and can be detected. Then, from this decay information, a tomographic image can be constructed.
Magnetization of the nuclear spins in the external magnetic field is a statistical process, which follows the Boltzmann distribution. Since the energy of the interaction between the nuclear spins and the static magnetic field as compared to the thermal energy at room temperature is relatively small, the overall magnetization by the static magnetic field is likewise relatively small, at the expense of the NMR signal.
One method to increase sensitivity of the NMR consists in increasing the strength of the static magnetic field, whereby a less uniform occupation of the nuclear spin states is achieved. However, there are technical limits to the strength of the magnetic field; typically, in state-of-the-art MRT devices, it is 1.5 T. With magnetic field strengths of more than 3.0 T, the patients can be moved into the magnet very slowly only, in order to minimize induced eddy currents, for example in the brain of the patient.
A further measure for increasing sensitivity consists in polarizing the sample more than what would correspond to the thermal occupation of the spin states in the given magnetic field. A sample, for which the occupancy of one or more spin states predominates compared to the other spin states clearly more than their energy difference according to the Boltzmann statistics would predict, is called hyperpolarized.
In MRT, it is known to administer a hyperpolarized fluid to a living being to be examined, i.e. a patient or an animal to be examined, which fluid generates NMR signals enhanced by several orders of magnitude. On the basis of the terminology from X-ray diagnostics, such hyperpolarized fluid is also designated as “contrast agent”. Predominantly, gases are used as contrast agent. However, it is also known to inject hyperpolarized liquids into the living being to be examined.
A known technology to hyperpolarize the contrast agent represents the so-called dynamic nuclear polarization (DNP). For DNP, first, electron spins in an external magnetic field are polarized. With resonant excitations of the electron spins in the microwave range, the electron spin polarization can be transferred to the nuclear spins by means of a weak interaction between the electrons and the nuclei. The underlying mechanisms are known as Overhauser effect, solid effect, cross effect, and so-called “thermal mixing”.
Herein, it is common that the contrast agent is polarized in its frozen state and in a relatively strong static magnetic field of, for example, 3.35 T. Under these conditions, the nuclear spins can be polarized considerably stronger than in the liquid state. However, in order to be able to administer the hyperpolarized contrast agent, first, it must be melted and transported to the patient. Then, the problem occurs that a substantial part of the hyperpolarization is lost due to relaxation processes upon transport. Furthermore, the method known from the state of the art is relatively complex.